The present invention generally relates to passive components having an optical isolator function independent of polarization to be used in optical fiber communications, and an optical isolator function for constituting an optical fiber amplifier.
Generally, the optical fiber amplifier inputs the rays of a laser diode of 1480 nm in wavelength into, for example, erbium doped optical fibers so as to excite erbium atoms to a high energy level. When signal lights of approximately 1550 nm in wavelength are inputted into the erbium doped optical fibers, the stimulated emission and radiation of the light equal in wavelength proportional to the size of the signal light is caused so as to amplify the signal lights along the erbium doped optical fibers.
Conventionally, the optical fiber amplifier is composed of a polarized light coupler for obtaining approximately twice as many optical inputs as optical outputs through the polarized coupling of 1480 nm wavelength lights of two pumping laser diodes, an optical multiplexer for inputting the pumping lights into the erbium doped optical fibers, and also, obtaining the lights of approximately 1550 nm in wavelength amplified by the erbium doped optical fibers, a polarization-independent optical isolator for making the light signals transmit only in one way direction so as to prevent the signal lights from being inputted into the erbium doped optical fibers again, because the amplified signal lights of approximately 1550 nm in wavelength are reflected because of some causes, and the optical fibers are advanced in the opposite direction. These optical passive components have optical fibers provided one by one on both the sides of two opposite lenses, with various types of optical materials disposed among the lenses so as to have passive functions. For example, the polarization-independent optical isolator has two crystals, a birefrangible (birefringent) crystal (for example, a rutile crystal) and a magnetooptical crystal (for example, a garnet crystal) provided between two opposite lenses so as to convert into collimated rays with a lens the lights inputted from the optical fibers for transmitting through the above described optical crystals. The polarization-independent optical isolator independent of polarization concentrates the rays with another lens so as to input them into the other optical fiber.
As the above described conventional optical passive components require two lenses or more per optical passive component, the components become more complex. As optical fibers are provided in, at least, two directions, there is a disadvantage that the shape of optical passive component including the draw-around space of the optical fiber becomes larger.
As the optical fibers of the respective optical passive components are in fused spliced connection with respect to each other when the optical fiber amplifier is composed with the use of the optical passive component, a connecting step is required, thus causing a loss increase in the rays because of connections. As space for securing many optical passive components and the accommodation processing space of the optical fibers after the connection are required, the optical fiber amplifier becomes larger.